There is a little icon on the bottom right of you screen. Press U and the swords cross, this means you are flagged for PVP and other players can attack you. Pressing U again unflags you and nobody can PVP you.So if the NDA is lifted (haven't played yet) how much solo PvP are you forced to do? If the territory holding is PvP centered I assume that is group related. and if there is an auto flag for certain missions I take it you are sometimes not set for PvP at all? mostly it is at end game and situational?
Doing PVP missions yeild almost double the XP or rewards in some cases, so if you want to level up fast, its the best path.
You can keep doing PvE as long as you like, although the current main story line tends to end around level 20.